Block

• There are two types of blocks

– Simple
– Guarded

Simple block

• The BLOCK statement, in its simple form, represents only a way of locally partitioning the code.

• It allows a set of concurrent statements to be clustered into a BLOCK, with the purpose of turning the overall code more readable and more manageable (which might be helpful when dealing with long codes).



• Block can be nested inside another block



Guarded block

• A guarded BLOCK is a special kind of BLOCK, which includes an additional expression, called guard expression.

• A guarded statement in a guarded BLOCK is executed only when the guard expression is TRUE.

Syntax

label: BLOCK (
guard expression)
[declarative part]
BEGIN
(concurrent guarded and unguarded statements)
END BLOCK label;

• Even though only concurrent statements can be written within a BLOCK, with a guarded BLOCK even sequential circuits can be constructed.

Latch
LIBRARY ieee;
USE ieee.std_logic_1164.all;
ENTITY latch IS
PORT (d, clk: IN STD_LOGIC;
q: OUT STD_LOGIC);
END latch;
ARCHITECTURE latch OF latch IS
BEGIN
b1: BLOCK (clk='1')
BEGIN
q <= GUARDED d;
END BLOCK b1;
END latch;

DFF
LIBRARY ieee;
USE ieee.std_logic_1164.all;
ENTITY DFF IS
PORT (d, clk, rst: IN STD_LOGIC;
q: OUT STD_LOGIC);
END DFF;
ARCHITECTURE DFF OF DFF IS
BEGIN
b1: BLOCK (clk’EVENT AND clk='1')
BEGIN
q <= GUARDED ‘0’ WHEN rst=‘1’ ELSE d;
END BLOCK b1;
END DFF;


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